70 DENNIS, ON THE MICROSCOPIC 



occurrence in fisli, seems to show that when they are present 

 it is for some other purpose than the transmission of blood 

 discs to the bone. Again^ why shoukl the lacunae be of dif- 

 ferent shapes even in the same bone ? Surely this can only 

 be explained upon the hypothesis of their connection with the 

 structure of bone, as that is connected with the life of the 

 animal. The canaliculi also seem to differ in size, as in the 

 femur and bone plates of the back of the armadillo. I think 

 the facts I have adduced tend to throw light, not only upon the 

 general structure of bone, but also upon the variation that 

 appears in fishes. In the fin-bone of the sturgeon the 

 lacunse are assembled in masses, so also in the dermal plates ; 

 and the bone of the plates is excessively hard for fishes' bone. 

 It would aj)pear that a combination of strength and tough- 

 ness is thus imparted to a greater degree than would be given 

 by any homogeneous mass of bone. The very princij)le has 

 been carried out in the tubular Menai bridge, which has 

 numerous cells or compartments, which add infinitely more 

 to the strength and stability of the structui'c than if it had 

 been a homogeneous mass ; and besides, by the dispensing 

 with superfluous matter, a reduction of specific gravity is 

 also obtained. For ordinary purposes, the gelatinous, homo- 

 geneous bones of fishes"^ seem sufficiently strong for their 

 mode of living, but a much more elaborate structure is 

 requisite in the higher Vertebrata. Only examine the fine 

 structure exhibited in the bones of the lion or the horse — 

 animals which require strength combined Avith lightness and 

 elasticity or resistance to violent shocks in their bones. The 

 canaliculi of the lion, which possesses great strength, and the 

 horse, which can carry or draw great burdens, are in both 

 numerous and fine. In the whalebone whale, which descends 

 to great depths, they are also numerous and fine; in the 

 elephant, where massive strength is required, they are not 

 numerous. In the Icthyosaurusf and P/esiosaurus, the cana- 

 liculi, though finer than those of some other saurians, are 

 few in number ; hence their bone is perfectly distinct from 

 that of mammifers or birds ; they have also, at least in parts, 

 straight Haversian tubes. Thus the whole structure of bone 

 is a portion of animal mechanism which, when thoroughly 



* Some have, as the cod, Haversian tubes without bouc-ccUs : they arc 

 seen vreW m the vomer. 



f Haversian tubes, or tlie semblance of them, I have only as yet found in 

 the jaw ; in the ribs it apjiears to be an extension of the cancellated structure, 

 of whieli, in fact, the bone of that animal seems to be composed. Tin- 

 small jiaddle-bones arc very curious, and have a singular complicated sot ol' 

 bone circles at their edges. The peculiar cancellated cavif ies I at first mis- 

 took for Haversian tubes. 



